The present invention relates to a sputtering target of sintered Ti—Nb based oxide, which has a high refractive index and a low extinction coefficient and is capable of depositing a thin film at a high rate; a thin film of Ti—Nb based oxide; and a method of producing the foregoing thin film.
In recent years, technology of high-density optical recording disks, which are high-density optical information recording media capable of rewriting without using a magnetic head, has been developed, and these optical disks are being rapidly commercialized. In particular, CD-RW has become the most widespread phase-change optical disk ever since its release as a rewritable CD in 1977. The rewrite cycle of a CD-RW is approximately 1000 times.
Moreover, DVD-RW for use as a DVD has been developed and commercialized, and the layer structure of this disk is basically the same as or similar to a CD-RW. The rewrite cycle of a DVD-RW is approximately 1000 to 10000 times.
These are electronic parts being rapidly spread that are used for recording, reproducing, and rewriting information by irradiating optical beams and optically changing the transmittance, reflectance and the like of the recording material.
Generally speaking, a phase-change optical disk that is used as a CD-RW, a DVD-RW or the like has a four-layer structure, in which both sides of a recording thin layer of Ag—In—Sb—Te, Ge—Sb—Te or the like are sandwiched between protective layers of high-melting-point dielectric such as ZnS and SiO2, and a reflective film of silver or silver alloy or aluminum alloy is further provided. Moreover, in order to increase the rewrite cycle, an interface layer is added between a memory layer and a protective layer as needed.
A reflective layer and a protective layer are required to have optical functions of increasing the difference in reflectance between the amorphous part and the crystal part of the recording layer, and additionally required to have humidity resistance of the recording thin film and a function for preventing deformation caused by heat, as well as a function of controlling the thermal conditions during recording (refer to Non-Patent Document 1).
Recently, in order to enable large-capacity and high-density recording, a single-sided/dual-layer optical recording medium has been proposed (refer to Patent Document 1). With Patent Document 1, there is a first information layer formed on a substrate 1 and a second information layer formed on a substrate 2 from the incident direction of the laser beam, and these layers are affixed to each other via an intermediate layer interposed therebetween so that these layers face each other.
In the foregoing case, the first information layer is configured from a recording layer and a first metal reflective layer, and the second information layer is configured from a first protective layer, a second protective layer, a recording layer, and a second metal reflective layer. In addition, layers such as a hard coat layer for protection against scratches, contamination and the like and a thermal diffusion layer may be arbitrarily formed. Moreover, various materials have been proposed as the protective layer, recording layer, reflective layer and other layers.
A protective layer of high-melting-point dielectric is required to be tolerant to the repeated thermal stress caused by heating and cooling so that the foregoing thermal effect does not affect the reflective film or other locations, and is also required to be thin and have low reflectivity and strength enough not to deteriorate. In this respect, the dielectric protective layer plays an important role. Moreover, needless to say, the recording layer, reflective layer, interference film layer and the like are similarly important in that they respectively exhibit their functions in the foregoing electronic parts such as CDs and DVDs.
The respective thin films in the foregoing multilayer structure are normally formed with the sputtering method. The sputtering method employs a principle where a substrate and a target as a positive electrode and a negative electrode are placed opposite each other, a high voltage is applied between the substrate and the target under an inert gas atmosphere to generate an electric field, the ionized electrons collide with the inert gas to form plasma, the positive ions in this plasma collide with the target (negative electrode) surface to discharge the constituent atoms of the target, and the extruded atoms adhere to the opposing substrate surface to form a film.
Under the foregoing circumstances, a target using titanium oxide (TiOx) has been proposed as a sputtering target for forming a heat reflecting film and an antireflection film (refer to Patent Document 2). Here, in order to stabilize the discharge during sputtering, the specific resistance value is set to 0.35 Ωcm or less so as to enable DC sputtering, and a film having a high refractive index is obtained thereby. Nevertheless, since the transmittance of the film is low, the measure of additionally introducing oxygen and achieving an oxygen content of 35 wt % or higher is adopted.
However, the introduction of oxygen entails the problem of deteriorating the deposition rate. Thus, attempts have been made for adding other substances in order to improve the deposition rate, but there were problems in the application as precision optics and electronic parts, for which films having a high refractive index and low absorption are required, particularly on the short wavelength side in the vicinity of 400 nm. Accordingly, the deterioration of the deposition rate in a titanium oxide target had not been resolved.
In addition, proposed is technology of forming a film essentially consisting of titanium oxide and niobium oxide or tantalum oxide as a dielectric film with high refractive index (refer to Patent Document 3). Nevertheless, in this case, the film is formed by using an alloy or mixture of titanium and niobium as the target, and performing (reactive) sputtering in an atmosphere containing oxygen gas. It is described that the refractive index of the obtained dielectric film with high refractive index is 2.5 or less. In the foregoing case, there are a problem in that stable film characteristics cannot be obtained due to the reactive sputtering, and a problem in that the extinction coefficient, which is deemed important in an optical recording medium, becomes high.
Meanwhile, also disclosed is technology of a titanium oxide-based (titanium oxide and niobium oxide) thin film having a refractive index of 2.5 or higher for use in an optical recording medium (refer to Patent Document 3). Here, a method of adding niobium oxide to reduce the resistivity and thereby enable DC sputtering is adopted, but this is still insufficient for usage as an optical recording medium.
In addition, there are patent documents that describe the combination among numerous oxides as to thin films for use in an optical recording medium (Patent Document 4, and Patent Document 5). These patent documents fail to describe the refractive index of the thin film for use in an optical recording medium, even though the refractive index is an important issue. Although various combinations are being considered, the refractive index will vary depending on the composition, and it is inferable that the refractive index has not been sufficiently examined. Moreover, it seems that, in these patent documents, a sputtering target is used to form the thin film, and the properties of the thin film are strongly affected by the component composition of the target and the nature of that target. However, these patent documents do not provide any disclosure thereof, and cannot be used as reference materials since they simply enumerate compositions and the disclosure of the technical contents is insufficient.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2006-79710    [Patent Document 2] Japanese Patent No. 3836163    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2002-277630    [Patent Document 4] Japanese Laid-Open Patent Publication No. 2003-13201    [Patent Document 5] Japanese Laid-Open Patent Publication No. 2004-158145    [Patent Document 6] Japanese Laid-Open Patent Publication No. 2009-157990    [Non-Patent Document 1] Technical Journal “Kogaku” Volume 26, No. 1, Pages 9 to 15